1. Field of the Invention
The invention relates generally to a method for producing high purity fused silica and doped fused silica soot blanks (or preforms) by thermal decomposition of precursors and deposition of the resulting oxides.
2. Background Art
High purity fused silica finds many diverse applications, e.g., as optical lenses for infrared transmission or as stepper lenses for deep-ultraviolet (UV) microlithography systems. However, a high purity fused silica having high water content is unsuitable for infrared transmission because water has some absorption at these wavelengths. Also, high purity fused silica has a small positive coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d) which makes it unsuitable for applications such as extreme ultraviolet lithography (xe2x80x9cEUVLxe2x80x9d). For example, if high purity fused silica is used as a mask blank for EUVL, heating of the mask blank as a result of absorption would cause the mask blank to expand. Such expansion will result in distortion and pattern placement errors at the wafer. This is a serious issue when using EUVL to print features at the sub-100-nm scale. For applications such as EUVL, doped glass is preferred. In particular, TiO2 -doped fused silica can be made to have a lower CTE than pure fused silica with the potential for a CTE that approximates zero.
Production of high purity fused silica or TiO2-doped fused silica generally involves transporting a silica precursor or a mixture of a silica precursor and a titania precursor to a reaction site, thermally decomposing the precursors, and depositing the resulting silica particles or TiO2-doped silica particles (xe2x80x9csootxe2x80x9d) on a support. The precursors may be in vapor form or may be vaporized and carried to the reaction site by a carrier gas. Originally, chlorides of silicon and titanium were employed as precursors. Recently, for environmental reasons, chloride-free precursors such as a siloxane, e.g., octamethylcyclotetrasiloxane (OMCTS), and a titanium alkoxide, e.g., titanium isopropoxide Ti(OPri)4, have been commercially employed. In the conventional boule process, the soot is continuously deposited, for example, in a cup of a refractory furnace where it melts to form a solid body (xe2x80x9cboulexe2x80x9d). Such boules may be used individually, may be finished and integrated together into large optical bodies, or may be cut into small pieces for finishing as lenses and the like. It is also possible to consolidate the soot into a solid body in a separate step.
Usage of organic precursors and a hydrogen-containing fuel in producing the soot inherently results in the fused silica containing more water than can be tolerated by infrared transmission applications or deep-UV applications such as at 157 nm. Another suspected problem is non-uniform deposition reactions during decomposition of the organic precursors at temperatures required to form the boule (typically 1500xc2x0 C. to 1900xc2x0 C.). For TiO2-doped fused silica in particular, the non-uniform reactions may result in large variations in the CTE of the glass. EUVL requires mask blanks with low variations in the CTE (e.g., 0xc2x15 ppb/xc2x0 C.).
In one aspect, the invention relates to a method for forming a fused silica glass. The method comprises delivering a silica precursor to a burner and passing the silica precursor through the flame of the burner to form silica particles and depositing the silica particles on a planar surface to form a flat, porous preform. The method further includes dehydrating the porous preform and consolidating the porous preform into a flat, dense glass.
In another aspect, the invention relates to a method for forming fused silica glass having a low water content. The method comprises delivering a silica precursor to a burner and passing the silica precursor through the flame of the burner to form silica particles and depositing the silica particles on a planar surface to form a flat, porous preform while maintaining the temperature of the planar surface in a range from approximately 600 to 900xc2x0 C. The method further includes dehydrating the porous preform and consolidating the porous preform into a flat, dense glass having an OH content below 1 ppm.
In another aspect, the invention relates to a method for producing a fused silica glass containing titania. The method comprises delivering a mixture of a silica precursor and a titania precursor to a burner and passing the mixture through the flame of the burner to form SiO2xe2x80x94TiO2 particles and depositing the SiO2xe2x80x94TiO2 particles on a planar surface to form a flat, porous preform. The method further includes consolidating the porous preform into a flat, dense glass.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.